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You start from a place where she is not attempting to limit, control or intervene in your relationship. You haven't actually started until she understands that. Until her insecurities are not dictating the nature of your relationship with her husband. Until she understands that sharing your body with her husband will not stop her being jealous.

Even if you are cool with not having sex alone with her husband, you don't want to start a precedent where her issues with sharing her husband consistently impact on your relationships. Unless you are happy with the emotional risk of being their guinea pig. Sexually and otherwise. If you are happy with the fact that she obviously has insecurities which may mean that she will pull out of this at any time, making it extremely difficult to continue your relationship with her husband.

You might say that anyone's spouse can pull out at any time, and that's absolutely true, that's why the majority of us try to get assurances from potential partners that they are poly, regardless of what their partner(s) may want from them, and then, we try to assure that their partner(s) are completely okay with them being poly too, just to lessen the chance of drama. Being completely okay with poly doesn't involve stopping your partner having a sexual relationship with someone else.